Modeling the transport of contaminants originating from the dissolution of DNAPL pools in aquifers in the presence of dissolved humic substances

Abstract

A two-dimensional finite difference numerical model was developed to describe the transport of dissolved organics originating from nonaqueous phase liquid (NAPL) pool dissolution in saturated porous media in the presence of dissolved humic substances. A rectangular NAPL pool was considered in a homogeneous porous medium with unidirectional interstitial groundwater velocity. It was assumed that dissolved humic substances and aqueous phase contaminants may sorb onto the solid matrix under local equilibrium conditions. The contaminant in the aqueous phase may undergo first-order decay. Also, the dissolved contaminant may sorb onto humic substances. The transport properties of dissolved humic substances are assumed to be unaffected by sorbing contaminants, because dissolved humic macromolecules are much larger than dissolved contaminants and sorp- tion of nonpolar contaminants onto humic substances do not affect the overall surface charge of humic substances. The sorption characteristics of dissolved humic substances onto clean sand were determined from column experiments. An effective local mass transfer rate coefficient accounting for the presence of dissolved humic substances was developed. Model simulations indicate that dissolved humic substances substantially increase NAPL pool dissolution, and consequently reduce the required pump-and-treat aquifer remediation time.